It has been considered to introduce an ultra-compact wireless base station (hereinafter, referred to as a “femtocell base station (HNB: Home Node B)”) into a cellular system represented by WCDMA (Wideband Code Division Multiple Access) or LTE (Long Term Evolution). The femtocell base station can cover an area of appropriately several meters in radius (hereinafter, referred to as a “femtocell”). The femtocell base station is installed within, for example, buildings such as ordinary households or offices having relatively bad propagation environment. In this manner, in the cellular system, a speeding up of wireless transmission within the femtocell is anticipated even in an area having bad propagation environment.
In existing cellular systems, it is assumed that a conventional wireless base station (hereinafter, referred to as a “macrocell base station (MNB: Macro Node B)”) uses the entire frequency band of telecommunications carriers in urban areas. Therefore, it is difficult to secure an exclusive frequency band for the femtocell base station. In this case, the femtocell base station is introduced in a form in which frequency is shared with an existing macrocell base station. Further, in an area where the femtocell base stations are densely installed, it is assumed that a plurality of femtocell base stations are operated by sharing a frequency. In addition, it is also assumed that an operation to which an access restriction is added is performed in a CSG (Closed Subscriber Group) mode capable of communicating using the femtocell base station by only subscribers or registrants of the femtocell base station.
When the femtocell base station is introduced into the existing cellular system on the basis of these conditions, there is a problem of mutual interference between downlink wireless lines provided for a wireless terminal (MUE: Macro User Equipment) connected to the existing macrocell base station from the femtocell base station and a wireless terminal (HUE: Home User Equipment) connected to the femtocell base station from the existing macrocell base station. Meanwhile, the wireless terminal connected to the macrocell base station is sometimes called a “macrocell user”, and the wireless terminal connected to the femtocell base station is sometimes called a “femtocell user”.
In addition, there is a problem of mutual interference between downlink wireless lines provided for a femtocell user (for example, a user of the femtocell base station installed on the adjacent house) who has not obtained access authorization from the femtocell base station. When such mutual interference between downlink wireless lines is generated, the effective communication range (effective coverage) of the femtocell base station is reduced, and the throughput of the downlink wireless line of the entire femtocell is lowered.
FIG. 14 is a diagram illustrating an example of a configuration of a wireless system in which a femtocell base station is installed within a coverage area of a macrocell base station (hereinafter, referred to as a “macrocell”). A macrocell base station (MNB) 100 forms a macrocell 101 that covers a wide area of which the cell radius is appropriately 0.5 kilometers to several kilometers. In the wireless system shown in FIG. 14, the macrocell user (MUE) 102 within the macrocell 101 communicates with the macrocell base station 100 using a wireless line such as WCDMA, LTE or WLAN. In addition, a femtocell base station (HNB) 110 is installed at an arbitrary position within the macrocell 101, and a femtocell 111 which is a range capable of communicating with the femtocell base station 110 is formed. When a wireless terminal which is an installer or a registrant of the femtocell base station 110 is located within the femtocell 111, and the reception quality of a downlink wireless signal from the femtocell base station 110 is above a certain level, the wireless terminal communicates with the femtocell base station 110 as a femtocell user (HUE) 112. In the example shown in FIG. 14, another femtocell base station (HNB) 120 different from the femtocell base station 110 is installed in the vicinity of the femtocell base station 110, and communicates with a femtocell user (HUE) 122 by forming a femtocell 121. Meanwhile, the macrocell base station (MNB), the femtocell base station (HNB), the macrocell user (MUE) and the femtocell user (HUE) communicate with each other using the same communication system (WCDMA, LTE, WLAN and the like). Meanwhile, an LTE-capable macrocell base station is sometimes called “MeNB”, and an LTE-capable femtocell base station is sometimes called “HeNB”. In addition, a wireless base station is sometimes called an access point (AP).
In NPL 1, an interference control method of a downlink wireless line in a femtocell base station is disclosed as a method of reducing mutual interference when the femtocell base station shares frequency with the existing macrocell base station. In this interference control method, transmission power of a femtocell base station (HNB) is set on the basis of the range of a coverage hole (area in which a macrocell user (MUE) is not capable of communicating with a macrocell base station (MNB)) of a macrocell formed by the femtocell base station and the range of a target coverage of the femtocell base station (HNB) itself.
FIG. 15 is a sequence diagram of the interference control method disclosed in NPL 1. As shown in FIG. 15, a notification signal (BCH: Broadcast CHannel) and a reference signal (RS: Reference Signal) are periodically transmitted from a macrocell base station (MNB) 202 toward the entire macrocell (ST201), and a femtocell base station (HNB) 201 installed within the macrocell receives the notification signal and the reference signal (BCH/RS). The femtocell base station (HNB) 201 measures reference signal received power (RSRP: Reference Signal Received Power) (ST202). Meanwhile, the reference signal is sometimes called “CPICH (Common Pilot CHannel)”. At this time, the reference signal received power is called “RSCP (Received Signal Code Power)”. The femtocell base station (HNB) 201 calculates a transmission power value of the femtocell base station (HNB) 201 (permissible transmission power value in relation to a macrocell) on the basis of RSRP measured in step ST202 and the range of a coverage hole permitted by the femtocell base station (HNB) 201 with respect to the macrocell (ST203). Specifically, the transmission power value of the femtocell base station (HNB) 201 is determined so that the reception quality of the reference signal by the macrocell user (MUE) from the macrocell base station (MNB) in a point (macrocell coverage hole) of which a propagation loss from the femtocell base station (HNB) 201 is X [dB] is above a certain value (for example, CPICH_Ec/No is −18 [dB] or more, or RSRP/RSSI is −6 [dB] or more). Meanwhile, “CPICH_Ec” indicates received energy per chip of CPICH, “No” indicates noise energy including an interference signal, and “RSSI (Received Signal Strength Indicator)” indicates total received power of a received signal. When RSRP of the macrocell user (MUE) is approximated by RSRP measured by the femtocell base station (HNB) 201, and RSSI approximates a value obtained by adding a propagation loss to transmission power of the femtocell base station (HNB) 201, the estimated reception quality of the macrocell user (MUE) can be calculated in the femtocell base station (HNB) 201. In order to satisfy the reception quality of the macrocell user (MUE), the transmission power of the femtocell base station (HNB) 201 is set to be small in the edge of the macrocell, and is set to be large in the vicinity of the macrocell. In addition, the femtocell base station (HNB) 201 calculates a desired transmission power value (femto desired transmission power value) of the femtocell base station (HNB) 201 from the range (for example, Y[dB]) of the target coverage of the femtocell base station (HNB) 201 itself and the RSRP measurement value of the macrocell (ST204). Next, the femtocell base station (HNB) 201 determines a femto operational transmission power value which is actually set, from the versus-macrocell permissible transmission power value calculated in step ST203 and the femto desired transmission power value calculated in step ST204 (ST205). Specifically, minimum values of the versus-macrocell permissible transmission power value and the femto desired transmission power value are selected. Thereby, while performing the interference reduction on the macrocell, it is possible to perform a control of the femtocell base station of which the coverage area is not excessively wide. The femtocell base station (HNB) 201 determines the transmission power of the notification signal and the reference signal on the basis of the determined femto operational transmission power value, and periodically transmits the notification signal and the reference signal (BCH/RS) to the entire femtocell (ST206).
In addition, NPL 2 discloses a further interference reduction method when an unregistered macrocell user in a femtocell base station enters a macrocell coverage hole. In the interference reduction method, a macrocell base station causes the femtocell base station to control transmission power by issuing power reduction request signaling to the femtocell base station which is an interference source, from a result of a received power measurement of a neighboring cell of the macrocell user receiving interference of a downlink wireless line from the femtocell base station.
FIG. 16 is a sequence diagram of the interference control method disclosed in NPL 2. As shown in FIG. 16, a notification signal and a reference signal (BCH/RS) which are specific to a cell are periodically transmitted from a femtocell base station (HNB) 301, a femtocell base station (HNB) 302 and a macrocell base station (MNB) 303, respectively (step ST301). Meanwhile, since each of the base stations has timing, the transmissions of BCH/RS are not necessarily at the completely same timing. A macrocell user (MUE) 304 measures each RSRP of the macrocell base station (MNB) 303 which is a serving cell and the neighboring cell detected by performing a neighboring cell search (step ST302). Meanwhile, the neighboring cell search performed by the macrocell user (MUE) 304 is based on a received result of a synchronization channel (SCH: Synchronization Channel), not shown, periodically transmitted from each base station using the downlink wireless line. The macrocell user (MUE) 304 reports the measurement result of RSRP of each cell to the macrocell base station (MNB) 303 which is a serving cell (step ST303). When the cell having the strongest RSRP except for the serving cell is a femtocell, the macrocell base station (MNB) 303 determines the other party's base station that issues an interference control request for asking the femtocell base station (HNB 302 in FIG. 16) for an interference reduction, on the basis of the reported measurement result (step ST304), and sends the interference control request (step ST305). Meanwhile, an identifier of the macrocell user (MUE) 304 may be included in the interference control request. The femtocell base station (HNB) 302 receiving the interference control request confirms a list of wireless terminals (hereinafter, referred to as a “CSG UE list”) providing access permission to a local station (step ST306). The femtocell base station (HNB) 302 reduces transmission power of the local station when the identifier of the macrocell user (MUE) 304 included in the interference control request is not included in the CSG UE list (step ST307), and maintains a transmission power value when the identifier thereof is included in the CSG UE list.
The following effects (1) and (2) are obtained by performing the downlink transmission power control of the femtocell base station based on a macrocell reference in this manner. As the effect (1), the coverage hole of the macrocell and the coverage of the femtocell can be formed to be almost the same as each other even in any of the places within the macrocell. As the effect (2), the femtocell base station in which the coverage hole is formed reduces the transmission power with respect to the macrocell user entering the coverage hole of the macrocell, thereby allowing the amount of interference of the downlink wireless line with the macrocell user to be reduced, and allowing the connection of the macrocell user to the macrocell base station to be maintained.